1. Field of the Invention
This invention relates generally to a gain control circuit, and more specifically is directed to a gain control circuit suitable for achieving contrast control in the reproduction of video images in television receivers, video tape recorders (VTRs) and the like.
2. Description of the Prior Art
In order to reduce the fluctuations in the contrast of reproduced video images resulting from variations in signal strength, receiver sensitivity, phasing, and the like, for example, in television receivers and VTRs, it is known to employ a gain control circuit including first and second clamping circuits, a variable gain amplifier interposed between the clamping circuits, a peak detector sensing peaks in the output of the second clamping circuit, and a comparator which compares the peak detector output with a predetermined voltage to provide therefrom a gain control signal applied to the variable gain amplifier. It is intended that the first clamping circuit be effective to clamp the pedestal level of the video signal at a reference level before effecting gain control, while the second clamping circuit is intended to achieve a similar clamping action after gain control is effected.
However, in the above described gain control circuit according to the prior art, the clamping circuits have fixed follow-up speeds so that, if such follow-up speed is even a little too high, noise superimposed on the input video signal or noise generated within the variable gain amplifier causes variations in the direct current level of the output video signal and consequent deterioration of the quality of the reproduced image. On the other hand, if the fixed follow-up speed of the clamping circuits is reduced in an attempt to overcome the above described problem, the image contrast changes with changes in the level of the received electromagnetic or video signal, and the effectiveness of the automatic gain control (AGC) function is reduced or even lost. In other words, if the clamping circuits have a fixed follow-up speed, such fixed follow-up speed will be either too high or too low in relation to the changing level of the input video signal.